The present invention broadly relates to electric alarms of the closed circuit type. The alarm systems of the closed circuit type trigger the alarm signal when any of the circuit-breaker detectors is opened or when the circuit-breakers connecting line is cut. This occurs because of the alarm relay, which is maintained normally energized, is deenergized and as a result its contact points operate for putting the alarm device in the circuit.
With the conventional alarm systems of the above closed circuit type there is the possibility that if an electrical bridge is established in the inlet of one of the circuit-breaker detectors, such circuit-breaker so bridged can be opened and no alarm signal will be produced at all. This occurs since the current in the closed circuit will continue circulating through the established bridge. The premises sought to be protected by such alarm systems therefore are likely to be tampered with, without the alarm system operating to give an alarm.
A conventional alarm of the closed circuit type normally comprises a plurality of detector elements, some of which may be located externally, or at least part of the feeding line interconnecting the detector elements is accessible from the outside. In some other cases, the detector elements, even though internally located, may still be susceptible of being electrically bridged, for instance if there are small openings such as spaces between rods of grates, through which the hands of non-authorized persons can be introduced in order to perform the bridging operation.
The present invention contemplates an electronic circuit in combination with the addition to the break-circuit detector elements of a closed circuit alarm system of an element of electrical resistance, whereby an electrical signal is produced in response to the electrical bridging of any of the detector elements of such closed circuit alarm system. This electrical signal is useful to energize the alarm device to protect the premises wherein the alarm system is set up.